1. Field of the Invention
This invention relates to electrically conductive adhesive webs and to methods of making them. In one embodiment, the invention relates to such web where the adhesive is normally tacky and pressure-sensitive, preferably aggressively tacky.
2. Description of the Related Art
Modern electrical devices are becoming so small that it is frequently virtually impossible to interconnect electrodes of two such devices either mechanically or by soldering. For many years there have been attempts to devise electrically conductive adhesives, and numerous U.S. patents have been issued of such products.
For example, U.S. Pat. No. 3,347,978 discloses electrically conductive joints and gaskets where an adhesive containing a mass of electrically conductive fibers is used to bond two metal members.
In the field of pressure-sensitive adhesives, U.S. Pat. No. 2,822,509 discloses an adhesive plaster having a conductive backing and an adhesive containing conductive particles. U.S. Pat. 4,606,962 describes a pressure-sensitive adhesive layer incorporating conductive metal-coated particles, which are somewhat thicker than the adhesive layer. U.S. Pat. No. 4,457,796 discloses a pressure-sensitive transfer adhesive containing chopped metal fibers to impart localized electrical conductivity.
Where it is important to have an adhesive that possesses both good electrical conductivity and good adhesion, most of the adhesive constructions discussed in the preceding paragraph have had serious problems. If a large quantity of conductive particles or fibers is blended with the adhesive, electrical conductivity is excellent, but adhesive properties are significantly reduced. Correspondingly, if a small quantity of conductive particles or fibers is used, adhesive properties are adequate, but the electrical conductivity is frequently too low to be acceptable. The problem is exacerbated where thick adhesive layers are required.
In a related nonadhesive development, U.S. Pat. No. 4,008,300 discloses a foraminous elastomeric film in which the perforations are filled with a hardened electrically conductive slurry that bulges beyond both faces of the elastomer. A major disadvantage of this film is that mechanical clamping is still required in order to maintain the electrical connections. When clamped between flat surfaces, lateral forcible expansion of the individual rods puts them under pressure, thus assuring that each rod electrically interconnects facing pairs of electrical terminals on the flat surfaces.
In certain industries, e.g., manufacture of heavy appliances and the like, metal panels are typically spot welded or riveted together to form the enclosure for the appliance. While these methods of joining the panels provide both mechanical strength and electrical continuity, skilled operators are required and the inevitable disruption of the metal surface requires an additional abrading step to restore a smooth surface. It has long been recognized that an aggressive adhesive construction having excellent electrical properties would be more convenient to use, but no such construction has actually been available.
It has now been discovered that an electrically conductive adhesive tape having good electrical properties may be formed using a very small amount of conductive particles which are contained in perforations in the tape.
It has also been discovered that such a tape can be made without sacrificing adhesive properties such as peel and sheer.